Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

will enable us to cascade async operations, which is necessary in order to
properly support DNS.  (For example, an HTTP request may have to redirect
to a new location and will have to perform a new DNS lookup, so we can't
just rely on doing the name lookup at the time of parsing the initial
URL).

Anything other than HTTP is probably broken right now; I'll fix the others
up asap.

order so we can just use buffer->fill.

Remove the now-totally-obsolete sockaddr_in field from tcp.h.

closed() method with a reason code.